For the purpose of improving the flying performance of golf balls, a variety of technical proposals were made from the past on the dimples of golf balls, particularly regarding the dimple distribution pattern and dimple configurations including dimensions such as diameter and depth. For example, U.S. Pat. No. 4,681,323 discloses the cross-sectional shape of dimples, U.S. Pat. No. 4,840,381 discloses the relationship between the cross-sectional shape and volume of dimples, and Japanese Patent Application Kokai No. 51871/1988 discloses the distribution of dimples. There are commercially available many golf balls to which these proposals are applied. However, there exists a demand for further improving the flying performance of golf balls.
The dimples on a golf ball play the role of assisting the transition of a boundary layer created in proximity to the ball surface due to motion and rotation of the ball from laminar flow to turbulent flow to move the point of separation rearward, thereby reducing pressure drag and creating a lifting force due to the difference of separation point between upper and lower positions of the ball. The separation point varies as various dimple parameters such as diameter and depth are changed. Thus the flying orbit of a golf ball is determined by a particular setting of dimple parameters.
Prior art efforts were concentrated on the diameter, depth, cross-sectional shape, and distribution of dimples as previously described. However, the situation is somewhat different with those golf balls having two or more different types of dimples though they have been marketed for some time. They are merely available as combinations of dimples having large and small diameters, but the same depth. No attention has been paid to the relationship between the diameter and the depth of different types of dimples. A mere combination of dimples having large and small diameters means that dimples having different aerodynamic properties are co-existing in a single ball, which deleteriously affects the flying performance of the ball.